There is a pressing commercial and industrial demand for ultra pure, dense, large particle size silica having physical properties essentially the same as natural sand.
Many researchers have produced synthetic silica. For example, monodispersed silica powder has been prepared by a number of researchers. The Stober process was based on the controlled growth of spherical silica particles of uniform size, W. Stober, A. Fink and E. Bohn, Journal of Colloid and Interface Science 26, 62-69 (1968). The process included hydrolytic decomposition of tetraalkyl silicates (methyl or ethyl) and subsequent condensation of silicic acid in an alcoholic solution. Ammonia was used as a morphological catalyst. Particle size obtained in suspension ranged from less than 0.5 micrometers to 2 micrometers in diameter. By modifying the hydrolysis conditions, B. Fegley produced monosized SiO.sub.2 powder with an average diameter of 0.55 micrometers, E. Barringer, N. Jubb, B. Fegley, R. L. Poker and H. K. Bowen, Ultrastructure Processing of Ceramics, Glasses, and Composites, 1984, pp. 315-333.
The polymerization of dilute silicic acid solution through nucleation and growth of silica particles in the presence of alkali salts or molybdic acid has been extensively studied by Iler and other groups, see R. K. Iler, Journal of Colloid and Interface Science, Vol. 75, No. 1, 1980, and "The Chemistry of Silica" by R. K. Iler, John Wiley & Sons Inc., (1979). Alexander and Iler reported the method of addition of alkalized acid sol, containing 2.4% silica to an alkaline sol containing 30% silica, in which the average diameter of silica particles increased to a final value of 60 micrometers from 45 micrometers in size.
U.S. Pat. No. 3,440,170 discloses a silica sol containing nonaggregated, uniform, spherical silica particles having a weight-average particle diameter of about 45-100 micrometers and a method of producing the sol by providing an alkaline silica sol containing silica particles having a weight-average diameter of about 10-30 microns.
U.S. Pat. No. 3,538,015 discloses the method to produce silica sols having water as the continuous phase and containing 30-70% by weight of uniform, nonaggregated silica particles having increased in diameter 2.5 to 4.0 times the original particle.
The aforementioned patents relate to the preparation of concentrated stable silica sols containing particles having a size from two to four times greater than those of original diluted sols. In addition, these sols contain trace amount of alkali metals or other foreign elements as stabilizers. The synthesis of ultra pure, dense, large particle size silica has not been reported.
In the present invention, we describe the method to prepare high purity, large particle size silica by hydrolytic decomposition of tetraalkylorthosilicates.